Environmental and Atmospheric Chemistry

Atmospheric chemistry explores the chemical processes that drive the formation and breakdown of compounds released from both natural sources and human activities. Various volatile organic compounds (VOCs) and volatile organosulfur compounds (VOSCs) that are released into the atmosphere play crucial roles, contributing to climate, acid rain, cloud formation, and effects on human health. Therefore, it is important to investigate the fate of VOCs and VOSCs in the environment to gain a more complete understanding of the reactivity and transformation mechanisms of these molecules.

In our research laboratory, we are focused on understanding the atmospheric fate of VOCs and VOSCs released into the Earth's atmosphere. The compounds of interest range from plant-derived alkyl thiosulfinates, alkanesulfinic acids, and alkanesulfinyl chlorides to ocean-derived dimethyl sulfone and methane sulfonamide. For example, we have recently embarked on studies of 3-methyl-2-butene-1-thiol, a volatile organosulfur compound emitted from Cannabis sativa, and chloroprene, a significant toxic air contaminant emitted from manufacturing industries.

To elucidate the atmospheric degradation mechanisms of these compounds, we investigate their primary oxidation pathways initiated by hydroxyl radicals (•OH), chlorine atoms (Cl), and nitrate radicals (NO₃). These pathways are examined through high level ab initio and density functional theory (DFT) calculations coupled with kinetic modeling. By optimizing the relevant minima and transition states on the potential energy surface (PES), we characterize all reaction pathways. Thermodynamic properties and accurate kinetic data for these reactions are subsequently derived.

Our research is divided into two primary domains to capture the full atmospheric lifecycle of these compounds:

  1. Computational Studies: We apply advanced computational methods to calculate kinetic parameters for the reactions of VOCs and VOSCs with OH radicals, Cl atoms, and NO₃ radicals. While experimental measurements yield overall rate coefficients, computational techniques allow us to quantify site-specific reaction contributions and differentiate between reaction types. Using transition state theory and variational transition state theory with tunneling corrections, we compute rate coefficients and estimate branching ratios for different reaction channels as a function of temperature.

    We further explore the reactions of key products generated in these processes with ground-state molecular oxygen (O₂), leading to the formation of RO₂ radical intermediates. These RO₂ radicals can undergo self-isomerization and subsequently react with NO and HO₂ radicals in the atmosphere. Using computational calculations and kinetic modeling, we assess the energetics and reaction kinetics, proposing the most plausible reaction mechanisms based on our findings.

  2. Experimental Measurements: The atmospheric lifetimes of these compounds are influenced by their reactivity with key oxidants such as OH, Cl, and NO₃. We use the relative rate (RR) method to measure kinetic parameters, comparing unknown reaction rates to well-characterized reference reactions under atmospherically relevant conditions.

This integrated computational-experimental approach provides a comprehensive understanding of the atmospheric processing and lifetimes of VOCs and VOSCs, aiding in the assessment of their environmental impact.

This movie shows: (A) a transition state obtained during the addition of OH radical at a terminal carbon atom of chloroprene; and (B) abstraction of a hydrogen atom by OH radical from the -CH site of the chloroprene.

Chloroprene (CP) emissions into the atmosphere can lead to the formation of toxic products such as HOCH₂C(OOH)(Cl)CH═CH₂, HC(O)H, HO₂ radical, ClC(O)CH═CH₂, HOCH₂C(O)Cl, HC(O) radical, Cl atom, and HOCH₂C(O)CH═CH₂. (Arathala, P. and Musah, R.A. J. Phys. Chem. A 2024, 128, 8983−8995)

The atmospheric removal of plant derived propanesulfinic acid (PSIA) initiated by •OH results in the formation of SO₂ from C–S single bond fission in the CH₃–CH₂–CH₂–S(O)₂ radical. The major products formed from this reaction are SO₂, propylene, sulfurous acid, and HO₂ radical. (Arathala, P. and Musah, R.A. ACS Earth Space Chem. 2021, 5, 1498−1510)

The release of methane sulfonamide (MSAM) from oceanic sources and its subsequent oxidation by Cl atoms, molecular oxygen (O₂), hydroperoxyl radicals (HO₂), and nitric oxide (NO) leads to the formation of atmospheric pollutants. These include sulfur dioxide (SO₂), formic acid (HCOOH), nitric acid (HNO₃), nitrous oxide (N₂O), carbon monoxide (CO), and carbon dioxide (CO₂). (Arathala, P. and Musah, R.A. ACS Earth Space Chem. 2023, 7, 1049-1059)